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Following on from the last post, ‘George’ was asking about mass-production and very small wind turbines. The impression was that he was thinking of extremely small ‘mills’, so for the sake of discussion, we’ll assume that something like the Eclectic Energy Stealthgen, or Marlec’s Rutland 903 is what he had in mind. These are 300 watt, lightweight HAWTs designed to mount on small yachts, lamp posts or road signs; I have seen a few used in this way in Oxfordshire, in conjunction with very small solar PV cells. As small turbines go, they are reasonably efficient for their size, and modestly priced (a few hundred pounds/dollars each). Being made of standard materials and of simple design, they are amenable to mass-production and could, in theory, be mounted in groups of fifty or a hundred in relatively small sites. Would this be a good idea?

To help explain why this would be less efficient (in both materials and productivity) than larger systems, a few very important basics of wind power need to be covered. I’ll skip some of the detail, but the principles are well-understood and already factored in to many manufacturing decisions.

The two most critical factors in the amount of power which can be generated from any wind turbine are the strength of the available wind (the wind energy potential) and the amount of wind ‘going through’ a turbine’s blades (the swept area). Other important factors include the ‘quality’ of the wind resource (energy-reducing elements such as shear and turbulence) and the siting of the turbine relative to surface conditions (surface roughness).

The greatest amount of wind energy potential anywhere is relative to height above ground level; the higher you go, the more energy is available. This is able to be calculated using the wind power law or the wind log law. Even a difference of a few inches can increase or decrease the average amount of wind received at the turbine head. And it is the average, or mean wind, which matters here; both in terms of directional stability and long-term unit productivity. This is why large wind turbines want to be sited on tall masts.

The ‘down-side’ of this factor is that effective (by which I mean cost-effective) installations are likely to be, by definition, visible in the landscape. Without going into the arguments here, I’d suggest that almost all of the objections (in the UK at least) to wind installations derive from people believing they will be visually intrusive or ‘ugly’. My simple response is that in many cases a single small or medium-sized turbine has considerably less visual impact than a radio mast or electricity pylon, and that, assuming that the aim of objectors is to preserve the countryside they cherish, the long-term view must be that, without renewable energy, the countryside is likely to be transformed in ways which are more permanent, more damaging, and more ‘ugly’ than almost any alternative.

The relation between swept area and power output is in a squared proportion. A turbine which sweeps 10% more area than an alternative will give perhaps 40% more power (I can’t be bothered to do the sums today) for the same wind energy available. This explains why the manufacturers of large systems, such as the ones used on wind farms, are focussing their production on ever-larger, ever more powerful turbines. In fact, most of the companies which once produced ‘mid-sized’ systems no longer make them at all. This is a matter which I’ll return to later, in discussing ‘affordable’ local solutions.

This does not mean that there is no place for small wind systems; on the contrary, ‘small’ wind could (according to recent estimates) account for a sizeable percentage of the entire UK’s ‘domestic’ energy use. It also does not mean that only large systems are economically viable. This is because, whilst wind farms needs to justify their costs by providing a return on investment and productivity at rates of 2-4 pence per kWh, small systems only need to work out as competitive in relation to end-user energy charges (tariffs), in order to be worthwhile. Having just seen a 13% rise in electricity charges yesterday, anyone who has energy generating capacity to replace that provided on the grid is making a net gain, since energy prices are rising faster than background inflation.

As things stand, with the technology currently available, (and depending on individuals’ current best available tariff), if a small turbine can produce 700-1100 kWh per £100 spent ($200), then it is probably going to both pay for itself and provide a long-term net return on the cost. If domestic users are willing to accept a small penalty in terms of cost in return for making a contribution towards reducing emissions, then the productivity can be even lower. Given the likelihood of ongoing long-term energy price inflation, a canny reader might realise that, so long as the additional cost is less than the sum of the inflation over the turbine’s lifetime, there is a reasonable chance that no cost will be incurred at all.

Rather than a field filled with extremely small turbines, then, I’d suggest that a chain of slightly larger ones (5-25 kW), is a more practical and more economic idea. In the next post or two, I’ll continue with more reasons why it works out this way.

In the meantime, if you have been thinking about putting a wind turbine up but aren’t really sure whether it is for you, or if you have a company which is suffering because of the high cost of energy, be warned that the Old man is now working for an engineering company in the UK which installs these things; get in touch, and I’ll try to help give you some answers (no charge, since it’s you…).



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January 2008